The present invention relates to a motor controller made up of a power supply circuit for rectifying an AC power source to output desired DC voltages while, at the same time, improving the power factor of the AC power source, and to a motor drive circuit for driving a motor.
A motor controller operating as a rectifying circuit for rectifying an AC power source to provide a DC power supply and for performing speed control of a motor by using, in combination, a power supply circuit, which is capable of suppressing harmonics occurring in the power source current, and a motor driving circuit is disclosed in Japanese Laid-open Patent No. Hei 6-105563 is known.
The motor controller is formed of a power factor improving converter circuit employing a step-up chopper circuit, which simultaneously performs suppression of harmonics in the power source current and control of the DC voltage, and an inverter circuit for driving the motor. The motor controller, at the time of low load operation, controls the DC voltage to take on the lowest voltage value which will allow the power factor to be improved and performs speed control of the motor through PWM control by the inverter circuit and, at the time of high load operation, stops the PWM control performed by the inverter and performs speed control of the motor through DC voltage control by the converter circuit, i.e., PAM control.
In the above described motor controller, the configuration of the motor speed control circuit at the time of low load operation and that at the time of high load operation is different, and so different speed controlling operations are necessary according to the conditions of the load. Namely, at the time of low load operation, the duty ratio of the PWM signal for the inverter is calculated from the speed deviation and, at the time of high load operation, the DC voltage command for the converter is calculated from the speed deviation.
Further, switching between the control circuit at the time of low load operation and the control circuit at the time of high load operation is carried out in accordance with the DC voltage value of the duty ratio of the PWM signal for the inverter, the speed command value and the present speed.
Thus, in the above described motor controller, it is required to have two kinds of speed control circuits, one for low load operation and another for high load operation, and hence, the control circuit becomes complex.
Further, since the determination of switching of the condition for the control circuits between the low load operation and high load operation is based on a number of different signals, many detector circuits are required.
An object of the invention is to solve the above described problems inherent in the known motor controller and to provide a motor controller capable of executing motor speed control by the use of one simple speed control circuit, whether the load is high or low.